Housing
What size is your tank? 2.5 gal
What temperature is your tank? It averaged about 65-70, but I got a heater and set it for 75
Does your tank have a filter? Yes, but I took it out because he would get sucked into it
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes, now but I just took it out because he was ok before I put it in and within 24 hours he is not looking well.
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? Aqueon betta pellets
How often do you feed your betta fish? 2x2-3 pellets per day

Maintenance
How often do you perform a water change? About weekly
What percentage of the water do you change when you perform a water change? 30-40%
What type of additives do you add to the water when you perform a water change? prime conditioner 2-3 drops/gal

Water Parameters:
Have you tested your water? If so, what are the following parameters? Not tested, but I have tester that shows if levels are ok and they are "non-harmful"

Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

How has your betta fish's appearance changed? - clamped fins
How has your betta fish's behavior changed? - Lethargic. He tries to eat but normally missed the food and won't try to get anymore after it drops. After one try, he'll ignore it, which isn't normal. 48 hours ago he would eat anything and everything, then ask for more.

First thing I noticed is you're not changing your water often enough. For that size of tank w/no filter you need to change it a minimum of twice weekly.. one 50% and one 100% but in my experience your fish will still be subjected to ammonia. Best thing to do until you feel confident about your fish in its environment is test daily with preferably drops kit until you know for sure the day ammonia starts showing up and do a partial change the day before.. plus at least the one 100% weekly. At this point you definitely need to get a kit and test it and be prepared to do changes. Any time you see .25ppm it needs a change and ideally you want to change before it reaches that point.

You might have heated up his tank too quickly and caused him shock, then you took the heater out and shocked him again when it dropped suddenly. You want to acclimate slowly. Usually I add a degree or two every hour or two. This is very slow but it is also 100% sure fire that there is no stress. Also tank heaters are seldom if never the correct temp. You need a tank thermometer (glass in tank) to determine actual temp.

Get the thermometer set up. Set the thermometer a few degrees below what his tank is at now and click it one degree up slowly until it turns on. This tells you the actual temp difference. From there increase slowly like above. Also did you rinse the heater? Anything that will be put in the tank needs to be rinsed in hot tap water first. Also sometimes cheaper heaters leach plastic and can poison fish. I've seen it quite a few times. What heater did you buy?

I do not think your fish needs meds. He is either suffering from shock and/or something leached from the heater and/or ammonia issues. I don't want to stress him too much by doing large water changes but you need to see where the ammonia is and if necessary be prepared to bring it down by doing several small changes. He really does need to be warmed up but I would do it very very slowly like above..after making sure the heater you have is not hurting him.